The image to the left uses all four of the tips that follow. 

Most lenses make slightly distorted images, whether they seem to bulge or contract in the center, and they can make parallel lines seem to waver.

Linear distortion like this is usually worse at the edges of the frame. Very wide-angle lenses are the worst culprits, but inexpensive telezooms can show noticeable distortion at their long ends. Some lenses add vignetting, too. In some subjects, like those with lots of straight lines, perspective distortion introduced by a tilted camera can also be a problem.

Fortunately, there are lots of ways to correct these issues. One of the simplest, since Adobe Photoshop introduced it in CS2, is the Lens Correction filter.

Since the introduction of CS5, there’s an added bonus to the tool: Photographers can shoot a target to create lens correction profiles for individual camera and lens combinations, then use the profiles to apply automatic corrections to their images. If you don’t want to make your own profiles, you can download profiles other shooters have created, and then apply them to your pictures.

If the automatic filter doesn’t correct the whole problem, or if you can’t find a profile, you can still use the filter in manual mode. Here’s how to do both.